With the spread of a high-speed mobile communication service, a cloud service, a video distribution service, etc., the amount of traffic over a network has greatly increased. Therefore, a high-speed interface of 100 Gbps per wavelength is requested for a transmission device on a backbone circuit of a network. Furthermore, the study and the development of a technique of realizing 400 Gbps through 1 Tbps have been performed.
Digital coherent detection has recently attracted attention as one of the techniques for transmitting an optical signal exceeding 100 Gbps. A digital coherent receiver recovers optical intensity information and phase information from received light by coherent detection. The recovered optical intensity information and phase information are digitalized by an A/D converter, and demodulated by a digital signal processing circuit. The digital signal processing circuit is realized by, for example, a DSP (digital signal processor).
The waveform of an optical signal is distorted on an optical transmission line. That is, a waveform distortion occurs on the optical transmission line. The waveform distortion is caused by the dispersion (for example, chromatic dispersion) of an optical transmission line. The waveform distortion includes a nonlinear distortion caused on the optical transmission line. When a waveform of an optical signal is degraded, there is the possibility of increasing an error rate of recovered data.
Therefore, a digital signal processing circuit of a digital coherent receiver may have a function of correcting the waveform distortion of an optical signal. In this case, a waveform distortion correction circuit includes a plurality of dispersion compensators and a plurality of nonlinear compensators. The plurality of dispersion compensators and the plurality of nonlinear compensators are alternately connected. That is, the dispersion compensating process and the nonlinear compensating process are alternately performed on a received optical signal. A set of dispersion compensator and nonlinear compensator may be referred to as a “stage”. If the number of stages increases, the compensation accuracy becomes higher.
A waveform distortion correction is performed using, for example, a training signal. In this case, a transmitter adds the training signal to a data signal. The data pattern of the training signal is known. Then, in a receiver (that is, a digital coherent receiver), the waveform distortion correction circuit performs the waveform distortion correction on the training signal. In this case, the parameters of the dispersion compensator and the nonlinear compensator are adjusted until the training signal is detected. When the training signal is detected, it is considered that the parameters of the dispersion compensator and the nonlinear compensator have been adjusted to the status close to the optimum values. Then, by finely adjusting the parameters of the dispersion compensator and the nonlinear compensator, the waveform distortion correction circuit is controlled into an appropriate operation state.
An optical transceiver having an electronic dispersion compensation circuit has been proposed as a related technique (for example, Japanese Laid-open Patent Publication No. 2008-54219). Also, an optical transmission system which efficiently improves the waveform distortion of an optical signal has been proposed (for example, Japanese Laid-open Patent Publication No. 2009-239555). Furthermore, a coherent optical receiver which may control tap coefficients of a digital filter and has a short tap coefficient setting time without using a chromatic dispersion measuring instrument has been proposed (for example, WO2009/060526). Furthermore, a device which detects the polarization dispersion and the chromatic dispersion of an optical signal has been proposed (for example, WO2009/144997).
As described above, a digital coherent receiver may correct the waveform distortion of an optical signal using a training signal added to a data signal in a transmitter. However, with the configuration in which a training signal is used, it is necessary to provide a circuit for processing a training signal in addition to a circuit for processing a data signal, thereby increasing the scale of a digital signal processing circuit. In addition, since a training signal is transmitted in addition to the data signal, it is necessary to increase a transmission rate of an optical signal. Thus, the higher the speed of an optical device, the more the cost increases. Furthermore, with an increasing speed of the optical signal, the transmission characteristic becomes easily degraded.
The problem may be solves by, for example, adjusting the parameter of a waveform distortion correction circuit by blind equalization without a training signal. However, in the blind equalization, the digital coherent receiver does not know the data pattern of a transmission signal. Therefore, it may take a long time to converge the parameter of the waveform distortion correction circuit, or a parameter of the waveform distortion correction circuit does not converge.
The adjustment of the parameter of the waveform distortion correction circuit is performed not only at the start of a communication service but also when an optical path is switched by a fault etc. Therefore, the adjustment of the parameter of the waveform distortion correction circuit is to be completed in a short time.